1. Field of the Invention
This invention relates to an improvement in a zoom lens small in size and weight, having an aperture ratio of about 2.0 and a variable power ratio of about 6 and suitable for video cameras or still cameras.
2. Description of the Prior Art
Four-group zoom lenses are known with a variable power ratio of about 6 wherein six lenses constitute a fourth group that performs an imaging function. For example, zoom lenses disclosed in Japanese Patent Laid-Open Nos.60-230112, 62-91908, and 1-170911 are of this type.
A tendency to use smaller image pickup devices is now common, and even 1/3 inch size image pickup devices are now being used. To design a zoom lens covering the standard view angle for such image pickup devices, it is necessary to reduce the focal length. However, since the thicknesses of the infrared cut filter and the optical low-pass filter cannot be substantially reduced, there is a need to maintain a sufficiently long back-focal distance. That is, it is necessary that the focal length is reduced while the back-focal distance is relatively increased.
It is difficult to reduce the focal length while maintaining a desired back-focal distance. For example, the problem of an increase in the length of the fourth group of lenses or an increase in the aperture of the groups of lenses is encountered when the desired back-focal distance is maintained.